Data converters, both digital-to-analog converters (DACs) and analog-to-digital converters (ADCs), are ubiquitous in applications involving digital signal processing of real-world signals, such as those found in communication systems, instrumentation, and audio and video processing systems.
As the ADCs may have errors, and the operation of the ADCs may drift with time, calibration is needed. Conventionally, the calibration of the ADCs is performed using more accurate ADCs, which are typically slower than the ADCs to be calibrated. To achieve an accurate calibration, the input signal should be high enough to fully calibrate all stages of the ADCs. However, even with the more accurate ADCs and high input signals, non-linear errors may not be able to be fixed through the conventional calibrations.
For example, FIG. 1 illustrates the residue transfer curve of a 1.5 B pipeline ADC, wherein the x-axis Aj represents the input signal of a stage of the ADC, while the y-axis Aj+1 represents the output signal of the stage. The stage represented by the residue transfer curve has an amplifying rate of 2, which means that the output signal is amplified to twice the residue voltage of input signal Aj. FIG. 2 illustrates an ideal case of the digital output code of the output signal AJ+1. It is noted that the digital output code Do is linear relative to the input signal Aj.
FIG. 3 illustrates a non-ideal residue transfer curve, in which the amplifying rate of a respective stage is deviated from 2. As a result of the deviation of the amplifying rate, the digital output code becomes non-linear, as shown in FIG. 4, in which gaps appear between different portions of the output. The output of the respective ADC thus becomes non-linear. Such problem cannot be solved by conventional ADC calibration regardless how accurate the calibration ADCs are. What is needed, therefore, is a method and structure for overcoming the above-described shortcomings in the prior art.